Mems device having a rugged package and fabrication process thereof

ABSTRACT

A MEMS device formed by a substrate, having a surface; a MEMS structure arranged on the surface; a first coating region having a first Young&#39;s modulus, surrounding the MEMS structure at the top and at the sides and in contact with the surface of the substrate; and a second coating region having a second Young&#39;s modulus, surrounding the first coating region at the top and at the sides and in contact with the surface of the substrate. The first Young&#39;s modulus is higher than the second Young&#39;s modulus.

BACKGROUND Technical Field

The present disclosure relates to a MEMS (Micro Electro-MechanicalSystem) device and the fabrication process thereof.

Description of the Related Art

As is known, electronic apparatuses comprising MEMS devices, such asMEMS movement sensors, are increasingly widespread. For the correctoperation of such apparatuses, it is desired that MEMS devices arecapable of detecting movement variations in an accurate and precise wayin all operating conditions. Consequently, it is desirable for MEMSdevices to be sufficiently sturdy so as not to break even when they aresubjected to abrupt movements (for example, as a result of the apparatusbeing dropped or undergoing mechanical shock). Furthermore, it isdesirable that their performance not to be significantly affected by theabove abrupt movements.

In most cases, it is not desirable to increase the robustness of MEMSdevices by increasing their dimensions. In fact, MEMS movement sensorsmay be modelled as mass-spring systems, the resonance frequency thereofstrictly depends on the geometry of the mass-spring system. Since theresonance frequency is an important parameter for determining theperformance of the MEMS device, it is not desirable to improve therobustness of the MEMS device by modifying its dimensions since thiswould have an undesired impact on performance.

Consequently, known solutions for increasing robustness consist inproviding mechanical stoppers operating outside of and/or within theextension plane of the MEMS movement sensor.

For instance, the U.S. Pat. Pub. No. 2013/299923 describes amicromechanical accelerometer comprising a seismic mass and asemiconductor substrate (for example, silicon) having a referenceelectrode. In particular, the seismic mass is moveable perpendicular tothe reference electrode; moreover, the seismic mass comprises a flexiblestopper operating in the movement direction of the seismic mass.

In addition, to increase robustness, it is known to treat the substrateby carrying out a slow etching step so as to maximize the contact areain the event of abrupt movements.

Furthermore, it is known to package MEMS movement sensors in resinscapable of absorbing part of the acceleration due to the sharp movementsso as to increase further robustness of the MEMS device.

However, known solutions have some disadvantages.

In fact, if subjected to repeated mechanical shocks with highaccelerations, mechanical stoppers of a MEMS movement sensor undergogradual damage and failure, causing failure of the mechanical stoppersin the long run, which thus no longer protect the MEMS movement sensor.

This is demonstrated by tumble tests carried out on single MEMS devices.For this purpose, the tested MEMS devices are dropped on a granite slabwith different accelerations ā which depend on different variables, suchas the contact stiffness, the roughness of the contact surface, thecontact angle, the contact points or areas and the air resistance. Indetail, the acceleration ā acting on the package of the MEMS device uponimpact with the granite slab is analytically estimated by the knownHertz theory (Eq. (1)):

$\begin{matrix}{\overset{\_}{a} = \sqrt[5]{\frac{v_{imp}^{6}R}{\left\lbrack {m\left( {\frac{1 - v_{t}^{2}}{E_{t}} + \frac{1 - v_{d}^{2}}{E_{d}}} \right)} \right\rbrack^{2}}}} & (1)\end{matrix}$

where ν_(imp) is the speed of impact; R is the radius of the object, mis the mass of the MEMS sensor; ν_(t) and ν_(d) are the Poisson's ratiosof the granite slab and of the MEMS device, respectively, and E_(t) andE_(d) are the Young's modulus of the granite slab and of the MEMSdevice, respectively.

The Applicant has verified that, both by applying Eq. (1) and with theaid of Finite-Element Modelling (FEM) simulations, that a MEMS devicehaving a package of 2×2 mm² perceives an acceleration ā of approximately100,000 g in case of the apparatus dropping in standard conditions, fromapproximately one meter of height from the granite slab. These repeatedaccelerations may lead to malfunctioning or failure of the MEMS device,thus rendering it unusable.

This problem is particularly felt when handling the MEMS device beforeassembling the package (in particular, fixing the MEMS device to asupporting structure). In detail, when the MEMS device is picked up byan automatic picker machine arranged on a supporting surface on which itis fixed (pick-and-place operation), impacts that lead to markedaccelerations of the order, for example, of tens of thousands of g mayoccur.

BRIEF SUMMARY

Embodiments are directed to a MEMS device and a fabrication processthereof. In particular, the present disclosure relates to a MEMS (MicroElectro-Mechanical System) device having a rugged package and to thefabrication process thereof. More particularly, reference is madehereinafter to a packaging process that uses an injection moldingsystem. Moreover, hereinafter reference is made to MEMS devicescomprising one or more MEMS sensors capable of detecting movements (suchas accelerometers), without this implying any loss of generality.

In one embodiment, a MEMS device is provided, formed by a substratehaving a surface; a MEMS structure arranged on the substrate surface; afirst coating region, having a first Young's modulus, surrounding theMEMS structure and in contact with part of the surface of the substrate;and a second coating region having a second Young's modulus, surroundingthe first coating region and in contact with part of the surface of thesubstrate. The first Young's modulus is higher than the second Young'smodulus.

The MEMS structure may be electrically coupled to the substrate.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

For a better understanding of the present disclosure, an embodimentthereof is now described, purely by way of non-limiting example, withreference to the attached drawings, wherein:

FIG. 1 shows a block diagram of an injection molding system;

FIGS. 2 to 5 show, in cross-section, successive steps of the presentfabrication process a MEMS device; and

FIG. 6 shows the plot of a characteristic quantity of a coating regionof the package of the present MEMS device.

DETAILED DESCRIPTION

FIG. 1 schematically illustrates an injection molding system,hereinafter referred to as system 200.

In particular, the system 200 comprises a hopper 202, which supplies amaterial to be injected in solid form (for example, in the form ofpellets); an injector 204, provided with a heater and an injectionsystem (not illustrated); and a molding chamber 206, housing one or morewafers or devices to be processed and comprising one or more moldingmatrices (not illustrated).

In use, in the molding chamber 206, the aforementioned one or moremolding matrices are fixed to the wafer or to the device to beprocessed. In particular, the molding matrix or matrices have one ormore cavities, which define the desired shape for the element to bemolded on the wafer or on the device to be processed.

The hopper 202 supplies the material to be injected to the injector 204,which, through the heater, heats it up to the melting point (or, in caseof plastic materials, the point of vitreous transition). The injectionsystem of the injector 204 injects the molten material into the moldingchamber 206, in particular into the one or more cavities of the moldingmatrix or matrices; in this way, the one or more cavities of the moldingmatrix or matrices is/are filled with the material that will constitutethe element to be moulded.

Once injection is completed, still within the molding chamber, theinjected material is subjected to a curing step and starts to polymerizeand solidify so that the desired moulded element is obtained. When themoulded element has solidified, the molding matrix is removed.

FIGS. 2-5 show successive fabrication steps of a plurality of packagedand singulated MEMS devices (three whereof are illustrated in FIG. 5).In particular, the present fabrication process is obtained by using thesystem 200 of FIG. 1.

FIG. 2 illustrates a processing wafer 1 comprising a substrate 5 (forexample, a laminated substrate or a semiconductor substrate, such as asilicon substrate), having a surface 5A. In particular, the substrate 5is adapted for a package of an LGA (Land-Grid Array) type.

The substrate 5 carries, on the surface 5A, a plurality of MEMSstructures 10, such as three MEMS dice are illustrated in FIG. 2. Indetail, each MEMS structure 10 is electrically connected to thesubstrate 5 through a plurality of conductive tracks (not illustrated).

Each MEMS structure 10 comprises an ASIC (Application-SpecificIntegrated Circuit) 11, extending over the substrate 5, and a MEMSsensor 12, extending over the ASIC 11. In particular, the ASIC 11 ismade per se known manner and is electrically and directly connected tothe conductive paths of the substrate 5 and/or to the MEMS structure 10,in a per se known manner. The MEMS sensor 12 is a movement sensor, forexample an inertial sensor, such as an accelerometer or a gyroscope,obtained in a per se known manner. The ASIC 11 and the MEMS sensor aremade of semiconductor material, such as silicon, using standardsemiconductor processing techniques.

With reference to FIG. 2, the processing wafer 1 is subjected to a firstinjection molding step. To this end, the processing wafer 1 is arrangedin the molding chamber 206 of the system 200 of FIG. 1.

As illustrated in FIG. 3, a first molding matrix 20 is fixed on thesurface 5A of the substrate 5. In particular, the first molding matrix20 comprises first molding structures 20A (three whereof are illustratedin FIG. 3) that form respective first molding cavities 20B. The firstmolding cavities 20B have, for example, a frustopyramidal shape. Othershapes are, however, possible. Each molding cavity 20B is delimited by arespective molding structure 20A and by the surface 5A of the substrate5 and it is arranged at a respective MEMS structure 10 so that each MEMSstructure 10 is accommodated in a respective first molding cavity 20B,between the respective molding structure 20A and the surface 5A of thesubstrate 5.

A first coating material, of a polymeric type, such as resin (forexample, EME-G770HE manufactured by Sumitomo), supplied in solid form(for example, pellets) by the hopper 202 to the injector 204 of thesystem 200, is brought to an injection temperature T_(i) in a range, forexample, between 170° C. and 180° C. (for example, 175° C.) by theheater of the injector 204, to form a first molten polymericagglomerate.

The first molten polymeric agglomerate is injected into the moldingchamber 206 by the injection system of the injector 204, at a transferpressure p_(tr) in a range, for example, between 7 MPa and 12 MPa (e.g.,8 MPa). Injection of the first molten polymeric agglomerate leads to thefilling of the first molding cavities 20B of the first molding matrix20, and enables complete coating of the plurality of MEMS structures 10and of the surface portions 5A of the substrate 5 delimited by the firstmolding matrix 20, thus forming first coating regions 25.

A first curing step is carried out, wherein the first coating regions 25are brought to a first curing temperature T_(c1), for example in a rangebetween 170° C. and 180° C. (in particular, 175° C.) in a first curingtime t_(c1) of a duration a range, for example, between 70 s and 120 s(in particular, 90 s). The first curing step enables cross-linking ofthe polymeric bonds of the first coating regions 25, enabling atransition phase from the molten state to the solid state.

At the end of the first curing step, the processing wafer 1 is extractedfrom the molding chamber 206. Next, it is possible to carry out a firstpost-molding curing step, for strengthening the structure of the firstcoating regions 25. In particular, the first coating regions 25 areheated in dedicated ovens, external to the chamber 206, at a treatmenttemperature T_(pc) in the range, for example, between 170° C. and 180°C. (e.g., 175° C.) for a treatment time t_(pc) longer than the firstcuring time t_(c1), having a duration in the range, for example, between2 hrs and 8 hrs (in particular, 6 hrs). In this way, the polymeric bondsof the first coating regions 25 are further cross-linked, and hencestrengthened.

Alternatively, the first post-molding curing step is carried out in themolding chamber 206.

At the end of the above steps, the processing wafer 1 has a plurality offirst coating regions 25 that coat respective MEMS structures 10.

By virtue of the use of a polymeric material, in particular the aboveresin manufactured by Sumitomo, each first coating region 25 iscompatible with the materials of the substrate 5, of the ASIC 11, and ofthe MEMS sensor 12, so as to limit the residual stresses caused byinterfacing different materials. Moreover, in the present case, eachfirst coating region 25 has a Young's modulus in the range, for example,between 20 GPa and 30 GPa in standard conditions of temperature andpressure (i.e., at 25° C. and 1 atm).

With reference to FIG. 4, the processing wafer 1 is subjected to asecond injection molding.

In particular, after removing the first molding matrix 20, a secondmolding matrix 30 is arranged on the surface 5A of the substrate 5 ofthe processing wafer 1. The second molding matrix 30 comprises a secondmolding structure 30A, which covers the entire surface 5A of thesubstrate 5, and forms a second molding cavity 30B having, for example,a cylindrical shape. The second molding cavity 30B is delimited by thefurther molding structure 30A and by the surface 5A. Thus, the secondmolding cavity 30B accommodates the MEMS structures 10 and therespective first coating regions 25.

Next, a second coating made of polymeric material, such as rubber (forexample, Sylgard 567 manufactured by Dow Corning), is supplied in liquidform (in particular, in case of Sylgard 567, a first and a second liquidcomponent, mixed with each other) from the hopper 202 to the injector204 of the system 200. In particular, the injector 204, through theheater, brings or exposes the second coating material up to theinjection temperature T_(i). In this way, the second coating material ismolten (in particular, rendered plastic), to form a second moltenpolymeric agglomerate.

Next, the second molten polymeric agglomerate is injected by theinjection system of the injector 204 into the molding chamber 206, inparticular into the second molding cavity 30B, at the transfer pressurep_(tr). Injection into the second molding cavity 30B of the secondmolten polymeric agglomerate fills the second molding cavity 30B andcompletely coats the surface 5A and the first coating regions 25 of theMEMS structures 10, to form a coating mass 35.

Next, the coating mass 35 is subjected to curing step. In particular,the coating mass 35 is cured for a second curing time t_(c2), of aduration, for example, of 180 min, at a second curing temperatureT_(c2), for example equal to 70° C. Alternatively, the second curingtime t_(c2) is approximately 120 min and the second curing temperatureT_(c2) is approximately 100° C. In both cases, the curing process herealso enables cross-linking of the polymeric bonds of the coating mass35.

At the end of the second curing step, it is possible to carry out asecond post-molding curing step so that the polymeric bonds of thecoating mass 35 are further cross-linked, and thus strengthened. Thesecond post-molding curing step is similar to the first post-moldingcuring step previously described with reference to the first coatingregions 25.

By virtue of the used material and to the described treatment processes,the coating mass 35 is compatible with the substrate 5 and the firstcoating regions 25 so that the residual stresses due to interfacing arelimited. Moreover, the coating mass 35 has a Young's modulus lower thanthe Young's modulus of the first coating regions 25, for example between100 MPa and 5 GPa, e.g., 500 MPa, in standard conditions of temperatureand pressure (i.e., at 25° C. and 1 atm).

At the end of the first and second molding processes, a processed wafer50 is obtained, which (FIG. 5) is diced, in a per se known manner, so asto obtain a plurality of MEMS devices 100, each having an own firstcoating region 25 and an own second coating region 37, deriving fromdicing of the coating mass 35.

The MEMS devices 100 efficiently absorb the impacts and/or mechanicalshocks to which they could be exposed during their operating life andprotect the delicate internal structures (ASIC 11 and MEMS sensor 12).In particular, since each first coating region 25 has a high Young'smodulus (i.e., a low flexibility), the first coating regions 25mechanically protect and strengthen the internal structures, minimizingthe thermo-mechanical stress between the materials of the first coatingregion 25 and the internal structures, as well as the substrate 5.Moreover, since the material of the second coating region 37 has aYoung's modulus lower than the Young's modulus of the first coatingregion 25 (and hence more flexible), the second coating region 37 isable to efficiently absorb the impact caused by possible mechanicalshocks.

Thus, the first and second coating regions 25, 37 are designed so as todecouple the mechanical stresses deriving from an external impact andderiving from interfacing between the different materials forming theMEMS device 100.

In this connection, the Applicant determined the plot of theacceleration ā as a function of the Young's modulus of the secondcoating region 37 of one of the MEMS devices 100 obtained according tothe fabrication process described previously. This plot is shown in FIG.6 and denoted by the reference A. In particular, the plot A was obtainedanalytically from Eq. (1), the abscissae representing the Young'smodulus of the material of the second coating region 37 (term E_(d) ofEq. (1)) and the ordinates representing acceleration ā.

The Applicant noted that, by decreasing the Young's modulus of thesecond coating region 37, the acceleration ā significantly decreases.Consequently, the acceleration ā perceived by each tested MEMS device100 is lower than the impact acceleration ā perceived by a MEMS devicewithout the second coating region 37; moreover, the height of fall wherethe acceleration ā is equal to 100,000·g increases. Consequently, thesecond coating region 37 imparts the MEMS devices 100 a greaterrobustness.

The Applicant then conducted further reliability tests and tests on theoccurred adhesion of the second coating region 37, including a test ofmechanical stress as reliability test and a peeling test as an adhesiontest of the second coating layer 37. In the mechanical-stress test, atest wafer and a reference wafer were used.

Initially, the test and reference wafers were optically analyzed usingknown instruments (such as instruments of optical analysis, infraredanalysis, X-ray analysis, tomography or SEM analysis), so as to verifythe structural homogeneity thereof.

Next, the test wafer was indented with a needle probe having a tip witha diameter equal to, for example, 0.6 mm, for a testing time t_(t) equalto, for example, 96 hrs, to detect the penetration rate, as well as thepenetration limit, of the needle probe in the second coating region 37of the test wafer.

In the executed indentation tests, the Applicant noted that the needleprobe penetrates at a rate of 0.1 mm/s and reaches a penetration limitequal to 50% of the thickness of the second coating region 37; moreover,these results were obtained in any point of the second coating region37.

Next, the test wafer was again analyzed and compared at an optical levelwith the reference wafer so as to verify the presence or absence ofevident indentations in the second coating region 37 of the test wafer.The Applicant noted that there were no clear differences between thereference wafer and the test wafer after indenting the test wafer.Consequently, the second coating region 37 is able to efficientlyrespond to an external mechanical stress, minimizing the negativeeffects thereof; moreover, this characteristic is substantially presenton the entire surface of the aforementioned second coating region 37.

In the peeling test, here of a chemical type, in the beginning the MEMSdevice 100 under analysis was treated with chemical solutions of a knowntype, such as nitric acid.

Next, the Applicant attempted to detach the second coating region 37from the surface 5A of the substrate 5 and from the first coating region25 and noted that the second coating region 37 detached in an unevenway, tearing. This result implies that the second coating region 37,obtained according to the fabrication process described previously, hasa good adherence both to the surface 5A and to the first coating region25.

The present MEMS manufacturing device and the corresponding process havevarious advantages.

In particular, the presence of two coatings with different Young'smoduli enables the reduction of the negative effects of impacts and/ormechanical shocks, so that the MEMS device is sturdy, albeit in theabsence of stoppers that are subject to deterioration. In fact, asmentioned above, the first coating region 25 (less flexible) minimizesthe thermo-mechanical stress between the materials of the substrate 5and of the second coating region 37, and the second coating layer (moreflexible) is able to absorb the impact waves (and hence, the impactacceleration) generated by this impact. In other words, thethermomechanical stress between the materials and the stress derivingfrom an impact are decoupled by virtue of the greater flexibility of thesecond coating region 37 with respect to the first coating region 25.

In addition, the coating regions 25, 37 do not modify the electrical ordetection characteristics of the MEMS device 100, which thus has apractically unvaried performance.

Furthermore, the present fabrication process enables formation ofcoating regions with a good degree of adhesion, and thus thecharacteristics of the MEMS device 100 are not degraded over time.

Moreover, the present fabrication process is simple to implement.

Finally, it is clear that modifications and variations may be made tothe MEMS device and to the corresponding fabrication process describedand illustrated herein, without thereby departing from the scope of thepresent disclosure.

For instance, the MEMS sensor 12 of the MEMS device 100 may be of anytype.

Moreover, each MEMS structure 10 may comprise more than one MEMSmovement sensor 12.

In addition, the materials forming the first and second coating regions25, 37 may be different from the ones used in the fabrication processdescribed previously; in particular, the choice of the materials maydepend, for example, upon the application field and the geometry of theMEMS device.

The various embodiments described above can be combined to providefurther embodiments. These and other changes can be made to theembodiments in light of the above-detailed description. In general, inthe following claims, the terms used should not be construed to limitthe claims to the specific embodiments disclosed in the specificationand the claims, but should be construed to include all possibleembodiments along with the full scope of equivalents to which suchclaims are entitled. Accordingly, the claims are not limited by thedisclosure.

1. A MEMS device, comprising: a substrate having a surface; a MEMSstructure arranged on the surface; a first coating region having a firstYoung's modulus, the first coating region on the surface of thesubstrate and covering the MEMS structure; and a second coating regionhaving a second Young's modulus, the second coating region covering thefirst coating region, wherein the first Young's modulus is higher thanthe second Young's modulus.
 2. The device according to claim 1, whereinthe first Young's modulus is between 20 GPa and 30 GPa, and the secondYoung's modulus is between 100 MPa and 5 GPa.
 3. The device according toclaim 1, wherein the first coating region comprises a polymeric resin.4. The device according to claim 1, wherein the second coating regioncomprises a polymeric rubber.
 5. The device according to claim 1,wherein the MEMS structure is electrically coupled to the substrate. 6.The device according to claim 1, wherein the MEMS structure comprises:an ASIC die arranged on the surface of the substrate; and a MEMS sensordie arranged on the ASIC die and electrically coupled to the ASIC die.7. A process comprising: coupling a MEMS structure to a surface of asubstrate; forming a first coating region on the first surface and overthe MEMS structure, the first coating region having a first Young'smodulus; and forming a second coating region over the first coatingregion, the second coating region having a second Young's modulus,wherein the first Young's modulus is higher than the second Young'smodulus.
 8. The process according to claim 7, wherein the first Young'smodulus is between 20 GPa and 30 GPa, and the second Young's modulus isbetween 100 MPa and 5 GPa.
 9. The process according to claim 7, whereinthe first coating region comprises a polymeric resin.
 10. The processaccording to claim 8, wherein the second coating region comprises apolymeric rubber.
 11. The process according to claim 7, wherein formingthe first coating region comprise: arranging a first molding matrixhaving a first molding cavity on the surface of the substrate so thatthe first molding cavity covers the MEMS structure; injecting a firstcoating material into the first molding cavity to form the first coatingregion; and removing the first molding matrix.
 12. The process accordingto claim 11, wherein injecting the first coating material comprises:exposing the first coating material to a first injection temperature;and injecting the first coating material at the first injectiontemperature into the first molding cavity at a first transfer pressure.13. The process according to claim 12, comprising, after injecting thefirst coating material, curing the first coating material by exposingthe first coating material to a first curing temperature for a firsttime period.
 14. The process according to claim 13, wherein forming thesecond coating region comprises: arranging a second molding matrixhaving a second molding cavity on the surface of the substrate so thatthe second molding cavity faces and surrounds the first coating region;and injecting a second coating material into the second molding cavityto form the second coating region.
 15. The process according to claim14, wherein injecting the second coating material comprises: exposingthe second coating material to a second injection temperature; andinjecting the second coating material at the second injectiontemperature into the second molding cavity at a second transferpressure.
 16. The process according to claim 15, comprising, afterinjecting the second coating material, curing the second coatingmaterial, wherein curing the second coating material comprises exposingthe second curing material to a second curing temperature for a secondtime period.
 17. A method, comprising: arranging a plurality of MEMSstructures on a first surface of substrate; covering the plurality ofMEMS structures with a plurality of first coating regions, respectively,the first coating regions having a first Young's modulus; covering theplurality of first coating regions with a coating mass and forming apackaged wafer, wherein the coating mass has a second Young's moduluslower than the first Young's modulus; and dicing the packaged wafer toform a plurality of MEMS devices.
 18. The method according to claim 17,wherein the first Young's modulus is between 20 GPa and 30 GPa, and thesecond Young's modulus is between 100 MPa and 5 GPa.
 19. The methodaccording to claim 17, wherein the first coating region is a resin andthe second coating region is a rubber.
 20. The method according to claim17, wherein each MEMS device comprises a respective MEMS structure, arespective first coating region, and a respective second coating regionderived from dicing the coating mass.